Electro-optical materials are materials with optical properties that can be changed by electric field. There are two types of electro-optical materials with very similar principle of operation, the manufacturing process and precursors for which are conceptually different. It was discovered in the year 1985 that a polymer material with liquid crystal dispersed in it as microdroplets can have electro-optical properties. Such material is named Polymer Dispersed Liquid Crystal, or PDLC. (J. L. Fergason, Polymer encapsulated nematic liquid crystals for display as light control applications, SID Int Symp. Digest Technol. Papers 16 (1985)). PDLC materials are also manufactured industrially.
PDLC-materials are composite films, where the micro-dimensioned droplets of nematic liquid crystals are dispersed in the organic polymer matrix. The droplets of liquid crystal are located within the matrix randomly, and their diameter is close to the wavelength of visible light, which causes strong scattering of light in the visible spectrum. The liquid crystal droplet formation mechanism lies in the phase separation occurring in the homogenous mixture of polymer precursor and liquid crystal. Phase separation may be induced by polymerization, decreasing the temperature (temperature-induced phase separation), or by solvent evaporation from the solution (solvent-induced phase separation). PDLC materials are manufactured industrially, but their main shortcomings are insufficient photostability (e.g., decomposition of the matrix under the influence of UV radiation), limited selection of precursors and high cost.
PDLC materials deserve to be mentioned in the context of this invention only because in the course of searching for the alternatives to PDLC it was found in 1991 that materials with the similar principle of operation, in which the droplets of liquid crystals are located inside a solid matrix, can also be prepared by sol-gel method (D. Levy, C. J. Serna, J. M. Otón, Preparation of Electro-optical Active Liquid Crystal Microdomains by the Sol-Gel Process, Mat. Lett. 10(9-10) (1991) 470-476, patent ES2137065 Procedimiento de preparación de matrices vítreas con propiedades de cristales líquidos a temperature ambient, 1999).
GDLC-s are a class of hybrid electro-optical film materials, manufactured by sol-gel method, which consist of a solid inorganic glass or organically modified inorganic glass matrix and the liquid crystal microdroplets dispersed in it. GDLC material is a solid microemulsion, since it consists of mutually insoluble solid and liquid phases. Since the use of inorganic glass as matrix, the photostability is not a problem in the case of GDLC materials.
In PDLC and GDLC materials the electro-optical effect is induced by placing the material between the two electrically conductive transparent electrodes. Thin-films of indium tin oxide are conventionally used as the electrodes. When no voltage is applied to electrodes, then, depending on the temperature and forces between the molecules of liquid crystals and the matrix boundary surface, the orientation of molecules of liquid crystals is different in different micropores, however, if electric field of sufficient strength is applied, then the molecules of liquid crystal polarize electrically and change orientation until the average orientation of molecules coincides with the direction of the external field. The liquid crystal and glass matrix are selected so that the liquid crystal's ordinary refractive index would be very close to the refractive index of the glass matrix. In this case, when electric field is applied to the film, the refraction indices of glass matrix and the effective refraction index of liquid crystal coincide, so there is no refraction and scattering of light in the material, thus the material is transparent. When no electric field is applied to the material, then the orientation of the liquid crystal molecules in different droplets is different due to thermal energy and forces between liquid crystal molecules and the pore surfaces. In this case the direction of incident light is at different angles to the orientation of liquid crystal molecules in different droplets and the refraction index of the liquid crystal for a specific ray of light does not match the refraction index of glass matrix. Thus refraction occurs and light is scattered.
In order to maximize the optical contrast of the material, the refractive index of the matrix must be equal to the refractive index of the nematic liquid crystal in the direction of its isotropic axis. However, the last is as a rule considerably higher than a refractive index of usual silicate glass (˜1.43), which is obtained by using the most common sol-gel precursors—silicon alkoxides. Increasing the refractive index of the matrix to the necessary range is resolved by using the alkoxides of other metals, the oxides of which have a high refractive index, as the precursors, or mixing them with other alkoxides, for example, with silanes. One of the precursors used in mixture with silanes for increasing the matrix refractive index is titanium ethoxide Ti(OCH2CH3)4. (Win-Pin Chang, Wha-Tzong Whang, Jaw-Ching Wong, Electrooptic Characteristics of amino-gel-glass-dispersed liquid crystal and its matrix formation, Jpn. J. Appl. Phys. 34 (1995) 1888-1894, U.S. Pat. No. 5,702,636 Gel-glass dispersed liquid crystals). The disadvantage of this method is the fact that the material preparation process has to be carried out in a special chamber with controlled humidity. Barium ethoxide Ba(OC2H4OC2H5) is also used (M. Hori, M. Toki, Electro-optical properties of inorganic oxide/liquid crystal composite film by sol-gel process, Journal of Sol-Gel Science and Technology 19 (2000) 349-352). In this case, for obtaining liquid crystal droplets in solid glass matrix, first the porous matrix with the necessary refractive index was prepared, and liquid crystal was inserted into the pores by vacuum infiltration instead of obtaining liquid crystal droplets in the matrix directly through phase separation from a homogeneous mixture of alkoxide-based precursor and liquid crystal (as was done in the methods described above). Latter method is complicated, because it consists of many stages and needs the application of vacuum and is therefore relatively expensive. At the same time the mentioned technical solution is only a distant analogue of the present invention.
The technology of bringing the refractive index of the matrix to the necessary value is extremely complicated due to the fact that the reactivity of suitable compounds (alkoxides of titanium, hafnium, zirconium, tin, strontium and other similar metals) is very high. The rate of hydrolysis and polymerization reactions of these compounds is by one order of magnitude greater than the corresponding reaction rates of silicon alkoxides. As a result of high reaction rate, the liquid crystal phase separation process, which requires certain gelation speed of the material so that liquid crystal droplets of optimal size could be formed, is greatly disturbed. State of art in this technological field does not provide a solution to this problem, which could ensure the simplicity of the process, necessary for the industrial manufacturing of the material. The present invention solves this problem.
The refractive index of the material prepared by the sol-gel method may be increased also by adding compounds that contain aromatic rings into the matrix (M. Oubaha, R. Copperwhite, B. Murphy, B. Kolodziejczyk, H. Barry, K. O'Dwyer, B. D. MacCraith, Development of photo-patternable organo-mineral hybrid films from the sol-gel condensation of alkoxysilanes, Thin Solid Films 510 (2006) 334-338), for example using phenyltriethoxysilane as one of the precursors. Such approach is not appropriate in situations where it is necessary to achieve such dielectric constant, electrical conductivity or porosity value of the matrix material, which cannot be achieved by using alkoxides that contain aromatic ring. The present invention solves this problem.
Electro-optical effect can also be achieved without matching refractive indices of matrix and liquid crystal (WO2007104818A1 Preparation of variable-transmittance coatings and assembled GDLC electro-optical devices), but due to purely physical reasons the change of transmittance on applying electric field is then smaller than it could be, if the corresponding refractive indices were equal. In addition, the need for matching the ordinary refractive indices of liquid crystal and matrix is smaller, when the concentration of liquid crystal droplets is very high so that the distance between the liquid crystals droplets is less than the light's wavelength (Paul S. Drzaic, Liquid Crystal Dispersions, World Scientific, Teaneck, N.J., 1995) and the film thickness is small. In applications where these conditions are not met, bringing the matrix refractive index to a necessary value is of key importance.
From the application point of view a very important material parameter is the driving voltage that is needed for switching a film of unit thickness from the opaque state to the transparent state. One of the essential factors that affect the driving voltage is the anchoring force of liquid crystal molecules on the pore surface. Anchoring forces and consequently the driving voltage for given material is modified by chemical functionalization in which such precursors as organically modified alkoxides, the non-hydrolizable groups of which remain on the surface of pores (M. Zayat, D. Levy, Surface Organic Modifications and the Performance of Sol-Gel Derived Gel-Glass Dispersed Liquid Crystals (GDLCs), Chem. Mater. 15 (2003) 2122-2128 and the patent application WO2007104818A1 Preparation of variable-transmittance coatings and assembled GDLC electrooptical devices).
The driving voltage may also be decreased by increasing the electric conductivity and the dielectric constant of the matrix (Win-Pin Chang, Wha-Tzong Whang, Jaw-Ching Wong, Electrooptic Characteristics of amino-gel-glass-dispersed liquid crystal and its matrix formation, Jpn. J. Appl. Phys. 34 (1995) 1888-1894, M. Hori, M. Toki, Electro-optical properties of inorganic oxide/liquid crystal composite film by sol-gel process, Journal of Sol-Gel Science and Technology 19 (2000) 349-352).
The present invention describes a novel method for decreasing the driving voltage by enabling the decrease of surface roughness of the matrix encapsulating the liquid crystal droplets.
From the part of the essence of solved problem the closest solution (prototype) to the present solution is WO2007104818A1 Preparation of variable-transmittance coatings and assembled GDLC electrooptical devices, in which the gel-glass dispersed liquid crystal material is obtained by mixing hydrolyzable alkoxides that subsequently polymerize, water, acid and liquid crystal in the appropriate ratio. The critical points in controlling the preparation process in corresponding method are the amount of solvent and its composition, the choice of precursor compounds according to their reactivity and adding a very precise amount of nitric acid, which performs as a catalyst in hydrolysis and polymerization reactions. An essential disadvantage of this method is limited shelf-life of the sol. The latter is caused by the presence of the catalyst in the precursor even when the coating procedure is not performed. The short stability time of prepared precursor hinders the industrial manufacturing of the material, especially in the case when synthesis and coating procedures are not subsequently carried out in the same place. In the context of the given invention, the precursor that is used for coating is a mixture of the alkoxides, which is up to 70% hydrolyzed, contains one or several different solvents, whether resulting from the chemical reactions or specially added, liquid crystal, whether homogenously dissolved or in the form of emulsion or in the form of suspension, and different dopants (e.g. dyes, nanostructures). In the case of higher extent of hydrolysis the properties of precursor gradually worsen and applying the material onto the substrate as a uniform film becomes difficult due to too high viscosity of the precursor.
The present invention is applicable in mass production, for preparing surface coatings of electrically variable transmittance. Unlike the prototype, in the case of the given invention there is no limitation to the shelf-life of the precursor due to the presence of the catalyst (the acid). This can potentially make the manufacturing process more economical by making it possible to prepare larger batches of precursor.
With the purpose of elaborating an analogue to LCD, dye doped GDLC materials have been prepared (D. Levy, F. Del Monte, X. Quintana, J. M. Otón, Color Displays with Gel-Glass Dispersed Liquid Crystals, Journal of Sol-Gel Science and Technology 8 (1997) 1063-1066). The advantage of such display would be the absence of polarizers but this technology also suffers from a problem of dye contamination that has not been overcome yet. It is inevitable with this method that when one of the GDLC phases (matrix or liquid crystal) is doped with a dye, it is also in some extent present in the other (liquid crystal or matrix, respectively). Present invention solves this problem, enabling the addition of liquid crystal into the matrix in the form of solid particles, while the liquid crystal is previously mixed with a dye in a liquid phase.